Mass spectrometer



gld.; A

Nov. 21, 1961 w. M. BRUBAKER ETAL 3,010,017

MASS SPECTROMETER Filed .June 1. 1959 s sheets-sheet 1 Nov. 21, 1961 w.M. BRUBAKER ETAL.. 3,010,017

MAss sPEcTRoMETER Filed June l, 1959 5 Sheets-Sheet 2 Nov. 21, 1961 w.M. BRUBAKER ErAL 3,010,017

MAss sPEcTRoMETER Filed June l, 1959 5 Sheets-Sheet 3 Jaf- E. .9x/frINVENToRs NOV. 21, 1961 w. M. BRuBAKl-:R Erm. 3,010,017

MASS sPEcTRoMETER Nov. 21, 1961 w. M. BRUBAKER ETAL 3,010,017

MAss SPECTROMETER Filed June l, 1959 5 Sheets-Sheet 5 "limited Statesparent fornita Filed June l, 1959, Ser. No. 817,258 25 Claims. (Cl.Z50-41.9)

The present invention relates to a device for passing charged particlesthrough magnetic fields. More particularly, charged particles of aselected mass moving in a magnetic field having an intensity gradientmay be directed and focused either -with respect to angular dispersionin their direction of movement or with respect to both angulardispersion in their direction of movement and dispersion in theirvelocity of movement.

The movement of charged particles in a magnetic field is in a curveddirection. The movement of charged particles in an electrical field isin a parabolic path. When a magnetic and an electric field are combinedby crossing so as to be perpendicular to each other, charged particlesimmersed in these crossed fields move in a direction perpendicular tothe magnetic field and along the electric field.

A charged particle subjected to crossed electric and magnetic fieldsnormally moves in a cycloidal path. The average velocity Vector of thecharged particle in the direction perpendicular to the magnetic fields,v, is given by the ratio of the electric field, E, to the magneticfield, B, with units in the meter-kilogram-second system of measurement.

Generally, charged particles enter the crossed fields with a velocitydifferent from that given by E/B. Even if a particle enters the crossedfields with a velocity of E/B, its direction of movement is not normallyin a straight line mutually perpendicular to the electric and magneticfields. However, a particle with such characteristics of motion, thatis, moving in a direction perpendicular to the electric and magneticfields with an actual velocity of E/ B, continues to travel in thestraight line at the E/B velocity so long as the crossed electric andmagnetic fields remain constant. Furthermore, if both the electric andmagnetic fields change in intensity so that E/B remains constant, theparticle continues to move in the straight line.

lt has been found that in crossed electric and magnetic elds in whichE/B is a constant, particles moving in an angular direction slightlydeviating from a line mutually perpendicular to the electric andmagnetic fields periodically cross this line. The distance between suchcrossings for such angularly dispersed particles is nvm/L1B, where m isthe particle mass, q is the particle charge, and the particles have thesame mass and velocity.

lt has further been found that particles initially moving in a directionperpendicular to the crossed electric and magnetic fields at a velocityonly slightly differing from the constant E/ B follow a path whichdeviates from a straight line mutually perpendicular to the electric andmagnetic fields and periodically return to this line. rIhese points ofreturn or focus of velocity or energy dispersed particles have a spacingof 2mm/QB. Therefore, it is apparent, by comparing the distance betweenangularly dispersed particle focuses, nvm/QB, to the distance betweenvelocity dispersed particle focuses, 21rvm/qB, that there are twice asmany focal points for angularly dispersed particles within a givendistance as for like velocity dispersed particles.

inasmuch as the E/B ratio gives the vector of velocity of all chargedparticles along the line mutually perpendichil? Patented Nov. 2l, i361ular to the crossed electric and magnetic fields, like charged particlesemanating from a point source, Whether angularly dispersed or velocity(energy) dispersed, bunch or focus at those points corresponding to thevelocitydispersed focal points. Furthermore, those points correspondingto angular dispersion crossings which are not velocity dispersioncrossings correspond to the points at which the physical dispersionbetween the paths of the velocity dispersed particles is a maximum andthe physical dispersion between angular dispersed particles is aminimum.

There exists a need for a device which is operable to introducev chargedparticles into a magnetic tield. Since charged particles normally followa curved path in a magnetic field, such introduction utilizingconventional means is difficult and often impossible. For example, in adevice such as a mass spectrometer, an intense magnetic field exists inthe device itself, and radiating outwardly from the device is a fringingmagnetic field which has an intensity gradient roughly inverselyproportional to its distance from the device. The fringing magneticfield has this intensity gradient due to the magnetic permeability ofthe substance surrounding the mass spectrometer and its magnet. Thus,charged particles passing through this fringing magnetic field prior totheir introduction into the mass spectrometer tend to assume a circularpath which makes their introduction into the mass spectrometerdifficult. For this reason, the conventional practice is to initiallyutilize particles having a neutral charge. These neutral particles canpass through the fringing magnetic field at a relatively low velocitywithout experiencing path deviation caused by the fringing magneticfield, since they have no charge. When immediately adjacent the massspectrometer inlet aperture, the particles are ionized and subjected toan electric field and are accelerated. This electric field acceleratesthe charged particles and forces them into the mass spectrometer.

According to the present invention, an electrical field is set up whichis perpendicular to the fringing magnetic field and which varies inintensity with the variations in intensity of the fringing magneticfield.

ln its preferred embodiment, the invention comprises a device consistingof two field-forming electrodes between which an electrical potentialdifference exists, so as to form an electrical field therebetween. Thedevice is utilized to pass charged particles through a fringing magneticfield without the particles assuming a circular path.

The field-forming electrodes are positioned about an axis along whichcharged particles travel. These electrodes are shaped so as to divergeasymtotically from the axis and oriented so as to have an increasingdivergence as the fringing magnetic field decreases. With such aconfiguration, for a given electrical potential difference existingbetween the field-forming electrodes, the electrical field strengthalong the axis varies inversely with the amount of divergence of theelectrodes from the axis. The variation in strength of the electricalfield is therefore proportional to the variation in the fringingmagnetic field strength.

By properly shaping the electrodes with respect tothe strength of thefringing magnetic field, a constant E/B ratio between the electric fieldof the field-forming electrodes and the fringing magnetic field isobtained.

ln an alternate embodiment, the same result is achieved by positioning aplurality of electrode pairs adjacent each other about the axis. Thepotential difference between opposite pairs of the field-formingelectrodes is then selected so as to maintain a substantially constantE/B.

As is stated above, charged particles follow a substantially straightline path in a constant E/B field, with traidor? particles of like masspassing through angular dispersion and velocity dispersion focusingpoints. Also, the actual points of focus for a particular particle massare determined by the velocity and the mass of the particle. One or moreadditional electrodes are therefore utilized adjacent either end of thefield formed by the field-forming electrodes to control the velocity ofthe particles passing between the held-forming electrodes. Potentialsappro priate to give the charged particles the velocity required tofocus particles of a selected mass at` a desired point are then appliedto these velocity-control electrodes.

The intensity and intensity gradient of the electrical tield required tobe set up by the field forming electrodes in a given application of theinvention is determined by the intensity and intensitygradient of thefringing magnetic field. The intensity and intensity gradient of thefringing magnetic iield may be measured by any one of a variety ofconventional methods.

|In the preferred embodiment, the held-forming electrodes are shaped sothat the intensity of the electrical. 4'ield therebetween changes inproportion to changes in the intensity of the ringing magnetic field.The actual coniiguration of the electrodes is determined by the gradientof the intensity of the magnetic tield. The electrodes are constructedto diverge in proportion to the weakening of the magnetic iield. Theheld-forming electrodes of the preferred embodiment, which are utilizedto give substantially straight line motion to the charged particles,have heretofore been described as diverging. However, these electrodesmay also be considered to converge asymptotically to parallel lines asthe fringing magnetic ield strength increases. vllt is to be understoodthat, as is used herein, the word diverging refers to the physicalconiiguration of the electrodes with respect to the point of closestphysical proximity of the electrodes,l andincludes electrodes which canconversely be described as asymptotically converging.

ln the alternate embodiment of the invention, the required electric ieldis set up by applying Vappropriate potential differences betweenopposite pairs of electrodes so as to give a substantially constant E/Bfor the measured B of the `fringing magnetic lield.

Other methods or' setting up the required electrical field and fieldgradient will be apparent to those skilled in the art. For example, theelectrical iield and iield gradient can be set up by utilizingsemiconductor materials for the ield-forming electrodes. The eld is thenset up by the potential difference between opposite portions of theelectrodes, and the iield gradient is set up by the potential drop alongthe electrodes.

Cycloidal mass spectrometers, that is, mass spectrometers in which, dueto mutu-ally perpendicular electric and magnetic fields, ion beamstransverse cycloidal paths to a resolution point, are Well known.Cycloidal mass spectrometers may be divided into two classes: those inwhich the ion beam follows a curtate cycloidal path, and those in whichthe ion beam follows a prolate cycloidal path. A mass spectrometer ofthe former type is described in U.S. Patent No. 2,844,726, issued July22, 1958 to C. F. Robinson, and assigned to Consolidated ElectrodynamicsCorporation, the assignee of the present application. Such a massspectrometer is `also described in 27 Review of Scientiiic Instruments504 (1956).

Conventional cycloidal mass spectrometers have a comparativedisadvantage of being somewhat insensitive. This insensitivity is due tothe fact that the collector electrode utilized to measure the magnitudeof the ion beam is located Within the cycloidal tube. Therefore, thccollector electrode must be of comparatively simple configuration andconstruction in order to avoid disturbing the electric and magneticiields within the cycloid tube. This collector electrode isconventionally placed at the 360 point of the ion beam path, that is,the point at which `the ion -beam of a selected mass of chargedparticles constitutes an image of the ion beam of the selected mass ofcharged particles as they entered the cycloid tube.

Utilizing the present invention, the conventional co1- lector electrodewithin the cycloid tube is replaced by a resolution aperture or slit,through which pass ions of the beam having a selected mass, so as toaccomplish mass resolution of the ion beam. These ions then commence anadditional cycle of their cycloidal paths, identic-al to their liirstcycle insofar as the crossed electric and magnetic fields remainconstant.

The desired mass resolution of the ion beam having taken piace, so thatonly ions of the selected mass remain in the ion beam, the resolved ionbeam is removed from the cycloid tube. In order to prevent thedispersion of the resolved ion beam by the fringing magnetic iield, theresolved ion beam is passed through a charged particle deviceconstructed according to the invention. The charged particle deviceserves to cause the ion beams to move in a substantially straight linepath, and also focuses the ion beam.

The focused and resolved ion beam then is measured by a conventional ionmeasuring device. For example, an electron multiplier may be used. Suchan electron multiplier as is described in US. Patent No. 2,854,583,issued September 30, 1958 to C. F. XRobinson, and assigned toConsolidated Electrodynamics Corporation, the assignee of the presentapplication, may be utilized.

In most applications, Vthe particles to be analyzed are neutral incharge. Neutral charges can be introduced into the magnetic field of themass spectrometer through the fringing magnetic iield surrounding themass spectrometer Without having their paths affected by the fringingmagnetic field. Once introduced into the magnetic field of the massspectrometer, the particles are ionized by au electron beam. Massresolution of the charged particles then is accomplished.

If the particles to be analyzed in the mass spectrometer are alreadycharged, their introduction into the mass spectrometer through thefringing magnetic iield is diiii cult. If the charged particles are in abeam, the beam tends to lose its coherent nature on transmigration ofthe fringing magnetic field. In order to overcome this diiiiculty,thecharged particle device of the present invention is utilized inconjunction with mass spectrometers to ionize the particles to beresolved, if necessary, to pass the charged particles through the iieldgradient of the fringing magnetic field, and to focus the chargedparticles at the mass spectrometer inlet aperture with respect to eitherangular dispersion, or both angular and velocity dispersion. The focusedcharged particle beam is then introduced into the mass spectrometer inthe conventional manner.

The invention may be more readily understood by reference to theaccompanying drawing in which:

FIGURE l (parts (a) and (b) taken together) illustrates curtatecycloidal paths of charged particles in crossed electric and magneticiields;

FIGURE 2 (parts (a) and (b) taken together) illustrates the paths ofcharged particles of like mass moving in crossed electric and magneticelds which increase in intt'ensity while maintaining a substantiallyconstant E/B ra 1o;

FIGURE 3 (parts (a) and (b) taken together) illustrates the paths ofcharged particles of like mass moving in crossed electric and magneticiields which decrease in intensity while maintaining a substantiallyconstant E/B ratio;

FIGURE 4 is a sectional view of a device according to the inventionutilized to introduce particles into a strong magnetic iield through amagnetic field increasing in strength;

-FEGURE 5 is a schematic sectional View of a mass spectrometer employingthe device of the invention to remove charged particles from the massspectrometer through a fringing magnetic field; and

FIGURE 6 is a schematic sectional View of a mass spectrometer employinga device of the invention to introduce charged particles into a massspectrometer through a fringing magnetic .field yand a device of theinvention to remove charged particles from the mass spectrometer throughthe fiinging magnetic field.

Referring to FIG. im), cycloidal paths for three charged particleshaving the same velocity and of mass M, M-f-AM, M-AM are shown. A massresolving plate 10 has a 180 point aperture il and a 360 point aperture12 therein. Charged particles `from a source 13 follow curtate cycloidalpaths due to crossed electric and magnetic fields (not shown). Particlesof mass M follow path .t4 and pass through the apertures l1 and i2.These particles of mass M continue to follow a curtate cycloidal path solong as they remain in the crossed electric and magnetic fields.Particles f mass M-AM follow a curved path i'. These particles of massM-AM strike the resolving plate and impinge thereon. Similarly,particles of mass M-j-AM follow a curved path 16 and strike theresolving plate l0. The curved paths 15 and 16 are the 0 to 180 and 0 to360 portions, respectively, of curtate cycloids.

In FIG. 1(b), the curtate cycloidal paths of charged particles havingidentical mass but different velocities are illustrated. Theseparticles, from a source 13', move in crossed electric and magneticfields (not shown) and pass through the 180 aperture l1 of the massresolving plate 10. The particles continue in their curtate cycloidalpaths, and pass through the 360 aperture l2 in a true focus. The 360aperture 1 2 serves to mass-resolve the charged particle beam. Thesemass resolved particles commence a second curtate cycloidal cycle afterpassing through the 360 aperture l2.

In FIGURE 2, a device according to the invention for injecting chargedparticles into a strong magnetic field through a fringing magneticfield, that is, a magnetic field having a strength gradient, isillustrated.

FIGURE 2(a) illustrates the paths followed by charged particles in afringing magnetic field perpendicular to which an electric field issuperimposed so as to maintain a substantially constant E/B ratio alonga line located between two held-forming electrodes. A firstfield-forming electrode 2d is connected to a lirst variable electricalpotential source 2l. A second field-forming electrode 22 is connected toa second variable potential source 23. rl`he charged particles passingbetween the field-forming electrodes 2,0 and 22 differ from each otheronly in having angular dispersion.

Assume that the charged particles enter the electrical field formed bythe field-forming electrodes with a velocity of ElBl from a point ornarrow slit source aperture. Assume also that the crossed fields along aline or axis midway between the field-forming electrodes have a value ofElBl. Those entering the crossed electric and magnetic fields along theaxis between the field-forming electrodes 2) and 22 with no angulardispersion with respect to this axis follow a straight line path 24which coincides with the axis. Particles entering the crossed iieldsalong the axis, but whose motion is angularly dispersed from the axis,follow paths such as 25 and 26, thek particular path followed dependingupon the magnitude and direction of the angular dispersion with whichthe particle enters the crossed electric and magnetic elds. The eiect ofthe crossed electric and magnetic fields on the angularly dispersedcharged particles is such as to focus all of these particles at a firstangular dispersion focal point 29 and again at a second angulardispersion focal point 30. lt will be noted that the focal lengthbetween the point at which the particles enter the crossed iields andthe first focal point 29 is much greater than the focal length betweenthe first focal point 29 and the second focal point 30. This change infocal length is due to the fact that the focal length varies inverselywith B, the -fringing magnetic eld strength.

FIGURE 2(b) illustrates the paths followed by velocity dispersedparticles in the charged particle device. The charged particle deviceconsisting of lfirst and second electrodes 2f# and 22 and two potentialsources 2l and 23 is the same as is illustrated in FIG. 2(0). Also, thecharged particles whose paths are shown in FlG. 2(b) have the same massand charge as the particles whose paths are shown in FIG. 2(a). Thosecharged particles entering the crossed electric and magnetic fields onthe axis between the field-forming electrodes 20 and 2,2 at a velocityof ElBl follow the straight line path 24. Those particles entering thecrossed electric and magnetic fields at the axis, but whose velocity isgreater or less than ElBl follow paths 3l and 32. The field-formingelectrodes Ztl and 22 have a configuration which is determined by theintensity gradient o-f the magnetic field in which they are immersed.The most common gradient of such fields is a decrease in the intensitywith distance in an inverse square relationship. All of these velocitydispersed particles are focused at a velocity dispersion focal point 35,which corresponds to the second angular dispersion yfocal point 30 ofFIG. 2(0).

ln FIG. 3 a device according to the invention for extracting chargedparticles from a strong magnetic field through a fringing magnetic fieldis illustrated. The components illustrated in FIG. 3 are identical tothe components illustrated in PEG. 2.

in FIG. 3(0) the paths of angularly dispersed ions of like mass, charge,and velocity are illustrated. Those particles entering the crossedelectric and magnetic fields along the axis between the twofield-forming electrodes Ztl and 22 with a velocity of ElBl follow thestraight line path 24. rlhose charged particles entering the crossedelectric and magnetic fields along the axis, but with angular dispersionwith respect to the axis, follow paths 25 and 26. All of these velocitydispersed particles are focused at a iirst velocity dispersion focalpoint 29 and at a second velocity dispersion `focal point. 30.

ln PIG. 3(b), particles of like mass and charge entering the crossedelectric and magnetic fields lof the extraction device with velocitydispersion are illustrated. Those particles entering the crossed fieldsalong the axis etween the field-forming electrodes 20 and 22 followpaths such as 3l and 32. The field-forming electrodes 20 and 22 have aconfiguration which is determined by the intensity gradient of themagnetic eld in which they are immersed. All of these velocity dispersedions are focused at a velocity dispersion focal point 35 whichcorresponds to the second angular dispersion focal point 36' of FIG.3(a).

FIGURE 4 is a schematic sectional view of a charged particle device,together with velocity control electrodes and electrical potentialsources, such as may be used to ionize particles and inject the ionizedparticles into a strong magnetic field through `a fringing magneticfield. The fringing magnetic field is indicated by an arrow indicatingthe direction of increasing field intensity, and the strong magneticfield exists between two pole faces 62 (only one of which is shown indotted lines) of a mass spectrometer magnet, for example.

Nonionized particles indicated by the arrow l0 are ejected from achamber al. The nonionized particles pass through an electron beam 42.The electron beam ionizes the particles passing therethrough. T woaccelerating electrodes 43 and 44 accelerate the ionized particles inthe direction of a charged particle device 45. Alternatively, theionized particles may be accelerated and directed into the chargedparticle device by use of only a single accelerating electrode i3 or 44.The use of two accelerating electrodes enables the ions to be formed ina -low electrical intensity, -while still providing for a sufficientvelocity control to focus the charged particles with respect to a broadrange of masses, as will be subsequently described. Y

The charged particle device i5 consists of two iieldforming electrodes46A and del?, to which electrical potentials are applied so as to formthe appropriate electrical iield between the field-forming electrodes46A and 46B. During their passage between the field-forming electrodes46A and 46B, charged particles of the same mass have two focal points:an angular dispersion focal point for ions of common mass and velocitybut varying in initial angular direction of movement; and, a velocityfocal point for ions of the same mass having velocity dispersion.

A path indicated by the solid line 47 corresponds to the path of anionentering the charged particle device in a direction perpendicular to thecrossed ieldg .therein and moving at a velocity of B21/B1. lonsdiffering from ions following the path 47 only in angular dispersionfollow, for example, paths indicated by either of two lines i8 or 49,depending upon the direction of the angular dispersion. The three paths47, 48 and 49 meet at a tirst focal point 50, diverge therefrom, andagain -meet at a second focal point l. Ions having velocity dispersionwith respect to the ions following the straight line path 47 follow, forexample, paths indicated by either of two lines 52 or S3. The angularand velocity dispersed ions following the paths S2 and S3 and the ionsfollowing the straight line path 47 first meet at the second focal point5l. Thus, at the second focal point 51;, ions having a common ymass arefocused both as to angular dispersion and as to velocity dispersion.

At the tirst focal point Si?, ions having a common velocity are focusedwith respect to angular dispersion. However, the physical spread betweenvelocity-dispersed ions is a maximum at the focal point Sil. Thus, byselecting either the focal point 50 or the focal point 51 to coincidewith an inlet aperture Sli of an appropriate apparatus, all chargedparticles having a common mass can be injected into the apparatus;alternatively, a selected portion of those ions exhibitimy velocitydispersion with respect to the median ion path can be excluded from theapparatus. In addition, by changing the electrical iield of the chargedparticle device, while `maintaining the same fringing magnetic held, thespectrum of velocity dispersion may be scanned at the inlet aperture S4.

In order to select which of the two focal points will be utilized, andin order to accommodate a wide range of velocities of charged particles,a variable electrical potential source 55 is utilized. The variableelectrical poten-tial source 55, as illustrated in FIG. 4, consists or"a battery 56 and potentiometers 53, 59 and 66. The potentiometer 5S isutilized to adiust the potentials on the iieldforming electrodes 416Aand 46B.

The potential of the iinst accelerating electrode 43 is adjusted bymeans of potentiometers S9 and 60 to set up the proper electrical heldwith ionization of the particle stream dit by the electron beam e2.

A velocity cont-rol electrode 611 is located between the chargedparticle device 45 and the inlet aperture Se. The potential differencebetween the second accelerating electrode 44 an-d the velocity controlelectrode 61 is adjusted by means of potentiometers S8, 59 and 60 togive a selected velocity to the charged particles passing through thecharged particle device 45. r{"his velocity is selected so that therequired focal points of the charged particles are brought with-in thecharged particle device, since, as stated above, the focal points aredetermined by the velocity, v, and the mass, m, of the particle. Thus byutilizing a wide range of potential differences between the secondaccelerating electrode 44 and the isolating electrode 61, a wide nangeof masses of charged particles may be selectively focused by the chargedparticle device.

FlGURE 5 illustrates an embodiment of the invention in which massresolved ions are extracted from a cycloid tube at the 450 point of thecycloid path. A cycloid tube 70 has electrodes 7l to which electricalpotentials are applied in order to establish an electrical potentialgradient lor held across the tube in a direction perpendicular to amagnetic field B. Such tubes are wel-l known in the mass spectrometryart.

A charged particle device '72 is positioned about an outlet aperture '73for the mass-resolved ion beam. An electron multiplier 74 and aconventional mult-ipl-ier output measuring means 75 measure themagnitude of the masseresolved ion beam after its passage throughthecharged particle device 72 and focusing elect-rode 74.

A beam of particles or molecules (illustrated by a dotted line 76) isionized by an electron beam 77 and introduced into the cycloid tube 79through an inlet aperture 7:8 by any conventional means. For example, arepeller electrode 7-9 so charged as to deilect the ion beam into thecycloid tube may be utilized. The cycloid tube 70 contain-s an ionresolving plate lil having a 180 aperture l1' and a 360 aperture 12'.

Ions of a mass determined by the magnitude of the crossed electric andmagnetic iields pass through the apertures ll and 12' and continuethrough the ion beam outlet aperture 73 into the charged particle device72. Iihe ion beam outlet aperture '73 is positioned at the 450 point ofthe cuntate cycloida-l path followed by the resolved ion beam. The ionspassing through the 360 point aperture l2' are in focus at that point,as explained previously with respect to FIG. l(b). However, at the 450point of the path, the ions have diverged, due to velocity differences,and therefore the outlet aperture 73 is comparatively wide.

Tlhe charged particle device 72 serves to cause the ion beam to travelin a straight line path rather than in a cycloidal path. Thus theelectric and magnetic fields within the charged particle device are ofsuch a magnitude that the ratio of E, the electric field, to B, themagnetic iield, equals v, the velocity of the ion beam, all dimen sionsbeing in the m-ks system. The electrical field, E, is provided by anelectrical potential difference between the field-forming electrodes ofthe charged particle de` vice 72. The magnetic eld, B, is the fringingmagnetic field which exists adjacent the cycloid tube. Since the actualmagnetic eld surrounding a particular mass spectrometer is dependentupon the physical coniiguraton of the spectrometer and its magnet, thedimensions of the charged particle device 72 must be deter-mined withrespect to the particular mass spectrometer and magnet in order tomaintain the E/B ratio constant.

As previously stated, small variations in the velocity of the ionsexist. By adjusting the electrical potential difference between thefield-forming electrodes of the charged particle device 72, the fieldstherein are made to be of the proper magnitude so that E/B along theaxis equals the median resolved ion velocity. Those resolved ions havingdiffering velocities then oscillate about the median velocity ions asthe ion beam moves thnough the charged particle device. The ionsperiodically pass through velocity and angular dispersion focal points.

A focusing electrode serves to regulate the velocity of the ion beamemerging from the charged particle device 72. The ion beam is focusedeither with respect to angular dispersion or angular and velocitydispersion by a potential applied to the focusing electrode 80 so thatthe ions are in a narrow beam as they strike a iirst dynode 79 of theelectron multiplier 74. Each ion striking the first dynode 79 causes aplurality of electrons to be emitted therefrom. Each of these emittedelectrons strikes the next dynode of the multiplier, causing a pluralityof electrons to be emitted therefrom. By this process, the number ofelectrons emitted from the first dynode '79 is multiplied in thesucceeding stages of the electron multiplier 74. An electrical currentis thereby produced, the magnitude of which is proportional to thenumber of ions striking the first dynode 79. This electron multiplieroutput current is measured by the multiplier output meas.-

9 uring means 75 to indicate the current and thus the number of ionsstriking the rst dynode 79.

The mass spectrometer illustrated in FIG. is relatively compact.However, the positioning of the charged particle device at the 450 pointof the curtate cycloidal path may cause the crossed electric andmagnetic fields within the spectrometer to become non-uniform at pointson the curtate cycloidal path prior to the 360 point, the portion inwhich mass resolution occurs. Such non-uniformity in the crossed fieldscauses the mass spectrometer to give fallacious results. In order toovercome this difficulty, alternative configuration of a massspectrometer and charged particle device as shown in FIG. 6 may beutilized.

In FIG. 6, a cycloid tube 90 has electrodes 91, to which electricalpotentials are applied to establish an electrical potential gradient orfield across the tube. A magnetic field B perpendicular to thiselectrical gradient or field is established by magnets (not shown). Thecycloid tube 90 contains an ion resolving plate l0" having a 180aperture Il and a 360 aperture 12". A charged particle stream 92 isintroduced into the cycloid tube 90 at an inlet aperture 93 through thefringing magnetic field by a charged particle device 94.

A charged particle device 95 is positioned about an outlet aperture 96for the mass-resolved charged particle beam. The outlet aperture 96 ispositioned to correspond to the 540 point of the cycloid path followedby the mass-resolved particles. A focusing electrode 97 is positionedadjacent the outer extremity of the charged particle device 95. Anelectron multiplier '74 and a multiplier output means (not shown)measure the magnitude of the mass-resolved ion beam after its passagethrough the charged particle device 95 and the focusing electrode 97.

The operation of the mass spectrometer of FIG. 6 is similar to thatdescribed above with respect to FIG. 5. The beam of charged particles 92is introduced into the mass spectrometer of an inlet aperture 93. Thecharged particle device 94 is utilized to pass the charged particle beamthrough the fringing magnetic field in substantially a straight line.The dimensions and potentials of the charged particle device 94 areselected to give a focusing of charged particles of a selected mass atthe inlet aperture 93. The mass of the particles injected into the massspectrometer is selected by adjusting the velocity of the particles, asis described with respect to FIG. 4. The velocity or energy dispersionof the particles of the selected mass is then scanned by focusing thebeam on the inlet aperture of an angular dispersion focal point which isnot a velocity dispersion focal point. By varying the E/B ratio of thecharged particle device, the velocity dispersion spectrum may be sweptacross the inlet aperture 93.

Rather than extracting the ions from the cycloid tube at the 450 pointof the cycloid ion path, the mass-resolved ions are extracted at the 540point of the path. Positioning of the charged particle device 95 at this540 point removes the charged particle device 95 from the proximity ofthe unresolved ion beam. Therefore, any effects which the chargedparticle device 95 has upon the electric and magnetic fields in itsimmediate vicinity are not refiected back into the unresolved portion ofthe charged particle beam.

We claim:

l. A device for passing charged particles through a magnetic fieldhaving a field strength gradient comprising a first and a secondelectrode, each having a pre-determined configuration positioned so asto diverge from each other about an axis, a source of electricalpotential, means for applying a potential difference from said source ofelectrical potential between the said first and second electrodes toproduce an electrical potential gradient therebetween, and acceleratingelectrodes positioned adjacent the divergent ends of said first andsecond electrodes, a velocity control electrode positioned adjacent theends of the first and second electrodes remote from the divergent endsthereof, and means connected to said source of electrical potential forapplying a potential difference between said accelerating electrode andsaid velocity control electrode.

2. A device for injecting neutral particles into a magnetic fieldthrougha fringing magnetic field having a field strength gradient comprising asource of nonionized particles, a beam of electrons positioned so as tointercept and ionize at least a portion of the nonionized particles, afirst and field-forming second electrode, each having a predeterminedconfiguration and positioned so as to diverge from the other along anaxis, a source of a selected electrical potential difference, means forapplying the selected electrical potential dierence to said first andsecond electrodes to produce an electrical potential gradienttherebetween, said first and second electrodes being positioned so thatthe divergent ends thereof are adjacent the electron beam.

3. A device as defined in claim 2 and including velocity controlelectrodes positioned adjacent the ends of the first and secondelectrodes and an electrical potential difference applied between saidvelocity control electrodes.

4. A device as defined in claim 3 and including an ionization fieldforming electrode positioned adjacent the electron beam and anelectrical potential connected to said ionization field formingelectrode.

5. In a mass spectrometer, the combination of an ion beam, a first and.a second electrode, each having a predetermined configuration and eachpositioned so as to diverge from the other about an axis, a source of aselected electrical potential difference, means for applying theelectrical potential difference to said first and second electrodes soas to produce an electrical potential gradient therebetween, said firstand second electrodes being positioned so that the ion beam emergingpasses between the first `and second electrodes.

6. In a mass spectrometer, the combination as defined in claim 5 inwhich lthe mass spectrometer is of the cycloid tu e type and theresolved ion beam is removed from the cycloid tube at the 450 point ofits curtate cycloidal path.

7. In a mass spectrometenthe combination as defined in claim 5 in whichthe mass spectrometer is of the cycloid tube type and the ion beam isremoved from the cycloid tube at the 540 point of its curtate cycloidalpath.

.8. In a mass spectrometer having an ion beam inlet aperture, thecombination of an ion beam, a first and a second field-formingelectrode, each having a predetermined configuration and each positionedso as to diverge from the other about an axis, a source of a selectedelectrical potential difference, means for applying the electricalpotential difference between said first and second field-formingelectrodes so as to produce an electrical potential gradient, said firstand second electrodes being positioned so that the ion beam passestherebetween'and through the mass spectrometer inlet aperture;

9. In a mass spectrometer, the combination of an ion beam, means forintroducing the ion beam into the mass spectrometer, means for removingthe resolved ion beam from the mass spectrometer, a first and a secondfieldforming electrode, each having a predeterminedv configuration andeach positioned so as to diverge from the other about an axis, a sourceof a selected electrical potential difference, means for applying theelectrical potential difference to said first and second field-formingelectrodes so as to produce an electrical potential gradienttherebetween, said first and second electrodes being positioned so thatthe ion beam emerging from the mass spectrometer passes between thefirst and second electrodes, and means for measuring the ion beamsubsequent to its passage between the first and second electrodes.

l0. In a mass spectrometer, the combination of an ion beam, a cycloidtube adapted to provide for mass resolution of an ion beam entering thetube at an ion beam entrance aperture and following a curtate cycloidalpath therein at the 360 point of the path and a resolved beam outletaperture for removal of the resolved beam from the tube, means forintroducing the ion beam into the tube at the ion beam entrance apertureincluding a first and a second field-forming electrode, each having asubstantially inverse square configuration and each positioned so as todiverge from the other about an axis, a source of a first selectedelectrical potential difference, means for applying the first electricalpotential difference between said first and second electrodes so as toproduce an electrical potential gradient, said first and secondelectrodes being positioned adjacent said entrance aperture, a third andfourth field-forming electrode, each having a substantially exponentialconfiguration and each positioned so as to diverge from the other aboutan axis, a source of a second selected electrical potential difference,means for applying the second electrical potential difference betweenthe third and fourth electrodes so as to produce an electrical potentialgradient, said third and fourth electrodes being positioned so that theresolved ion beam emerging from the tube passes between the third andfourth electrodes, and means for measuring the resolved ion beamsubsequent to its passage between the third and fourth electrodes.

l1. In a mass spectrometer, the combination as defined in claim in whichthe resolved ion beam is removed from the cycloid tube at the 450 pointof its curtate cycloidal path.

l2. In a mass spectrometer, the combination as defined in claim 10 inwhich the resolved ion beam is removed from the cycloid tube at the 540point of its curtate cycloidal path.

13. In a mass spectrometer, the combination of a source of particles tobe analyzed, a cycloid tube adapted to provide mass resolution of an ionbeam entering the tube atan ion beam entrance aperture and following acurtate cycloidal path therein at the 360 point of the path and aresolved beam outlet aperture for removing the resolved beam from thetube, means for ionizing the particles, a first and a second electrode,each having a substantially inverse square configuration and eachpositioned so as to diverge from the other about an axis, a source of afirst selected electrical potential difference, means for applying thefirst electrical potential difference between said first and secondelectrodes so as to produce an electrical potential gradient, said firstand second electrodes being positioned adjacent the ion beam entranceaperture so as to separate the entrance from the ionizing means, wherebyat least a portion of the ionized particles pass between the first andsecond electrodes and enter the cycloid tube, a third and fourthelectrode, each having a substantial exponential configuration and eachpositioned so as to diverge from the other about an axis, a source of asecond selected electrical potential difference, means for applying thesecond electrical potential difference between the third and fourthfield-forming electrodes so as to produce an electrical potentialgradient, said third and fourth electrodes being positioned adjacent theresolved beam outlet aperture so that the resolved beam emerging fromthe cycloid tubeV passes between the third and fourth electrodes, andmeans for measuring the ion beam subsequent to its passage between thethird and fourth electrodes.

14. The combination as defined in claim 13 in which the resolved ionbeam is removed from the cycloid tube at the 450 point of its curtatecycloidal path.

l5. The combination as defined in claim 13 in which the' ion beam isremoved from the cycloid tube at the 540 point of its cui-tate cycloidalpath.

16. In a mass spectrometer, the combination of a source of particles tobe analyzed, a cycloid tube adapted to provide for mass resolution of anion beam entering the tube atan ion beam entrance `aperture andfollowing a curtate cycloidal path therein at the 360 point of the pathand a resolved beam outlet aperture for removal of the resolved beamfrom the tube, means for ionizing the particles, a first and a secondfield-forming electrode, each having a substantially inverse squareconfiguration and each positioned so as to diverge from the other aboutan axis, a source of a first selected electrical potential difference,means for applying the first electrical potential difference betweensaid first and second electrodes so as to produce an electricalpotential gradient, said first and second electrodes being positionedadjacent the ion beam entrance aperture so as to separate the entrancefrom the ionizing means, means including at least one acceleratingelectrode positioned between the ionizing means and the first and secondelectrodes for causing atleast a portion of the ionized particles topass between the first and second electrodes and enter the cycloid tube,a third and fourth field-forming electrode, each having a substantiallyexponential configuration and each postioned so as to diverge frorn theother about an axis, a source of a second electrical potentiafdifference, means for applying the second electrical potentialdifference between the third and fourth electrodes so as to produce anelectrical potential gradient, said third and fourth electrodes beingpositioned yadjacent the resolved beam outlet aperture so that theresolved beam emerging from the cycloid tube passes between the thirdand fourth electrodes, and means for measuring the ion beam subsequentto its passage between the third and fourth electrodes.

17. in a mass spectrometer, the combination as defined in claim 16 inwhich the resolved ion beam is removed from the cycloid tube at the 450point of its curtate cycloidal path.

18. In a mass spectrometer, the combination as defined in claim 16 inwhich the ion beam is removed from the cycloid tube at the 540 point ofits curtate cycloidal path.

19. In a mass spectrometer, the combination of a source of particles tobe analyzed, a cycloid tube adapted to provide for mass resolution of anion beam entering the tube at an ion beam entrance aperture andfollowing a curtate cycloidal path therein at the 360 point of the path'and a resolved beam outlet aperture for removal of the resolved beamfrom Ithe tube, means for ionizing the particles, a first and a ysecondfield-forming electrode, each having a substantially inverse squareconfiguration and each positioned so as to diverge from the other `aboutan axis, a source of a first selected electrical potential difference,means for applying the first electrical potential difference betweensaid first and second electrodes so as to pro-duce an electricalpotential gradient, said first and second electrodes being positionedadjacent the ion beam entrance aperture so as to separate the entrancefrom the ionizing means, means including at least one velocity-controlelectrode positioned between the first and second electrode and theionizing means and at least one velocitycontrol electrode positionedbetween the first and second electrodes and the ion beam entranceaperture for causing at least a portion of the ionized particles to passbetween the first and second electrodes and enter the cy cloidal tube ata selected velocity, a third and a fourth field-forming electrode, eachhaving a substantially exponential configuration and each positioned soas to diverge from the other about an axis, 4a source of a secondselected electrical potential difference, means for applying the secondelectrical potential difference between the third and fourth electrodesso as to produce an electrical potential gradient, said third and fourthelectrodes being positioned adjacent the resolved beam outlet apertureso that the resolved beam emerging from the cycloidal tube passesbetween the third and fourth electrodes, and means for measuring the ionbeam subsequent to its passage between the third and fourth electrodes.

20. In a mass spectrometer, the combination as defined in claim 19 inwhich the resolved ion beam is removed from the cycloid tube at the 450point of its curtate cycloidal path.

2l. In a mass spectrometer, the combination as defined in claim 19 inwhich the ion beam is removed from its cycloidal tube at the 540 pointof the curtate cycloidal path. t

22. A charged particle device for passing charged particles through amagnetic eld having a field strength gradient comprising at least twoopposed electrodes and an electrical potential applied to eachelectrode, in which said electrodes are positioned with respect to thefringing magnetic field so that the electrical field set up betweenopposed electrodes by the application of the electrical potentialsthereto is perpendicular to the magnetic field, and the electrical fieldstrength has a gradient which establishes a line through the fringingmagnetic field whose locus is defined |by a substantially constant ratioof electrical field strength to magnetic field strength.

23. A charged particle device as defined in claim 22, in which the linethrough the magnetic field is a substantially straight line.

24. A device for injecting charged particles into a magnetic fieldthrough a fringing magnetic field having a field strength gradientcomprising opposed electrodes each of which converges on an axis in asubstantially inverse square relationship as the fringing magnetic fieldstrength increases and a source of electrical potential 14 applied toeach electrode, whereby the ratio of the electric field strength betweenthe electrodes to the fringing magnetic field strength between theelectrodes remains substantially constant along the axis of convergenceof the electrodes.

25. A device for extracting charged particles from a fringing magneticfield through a fringing magnetic field having a field strength gradientcomprising opposed electrodes each of which diverges from an yaxis in asubstantially square relationship as the fringing magnetic fieldstrength decreases and a source of electrical potential applied to eachelectrode, whereby the ratio of the electric field strength between theelectrodes to the magnetic field strength lbetween the electrodesremains substantially con stan-t along the axis of divergence of theelectrodes.

References Cited in the file of this patent UNITED STATES PATENTS2,473,031 Larson June 14, 1949 2,780,729 Robinson Feb. 5, 1957 2,794,126Robinson May 28, 1957 2,806,956 Hall Sept. 17, 1957 2,880,356 Charles etal May 31, 1959 UNITED STATES PATENT oEEIcE CERTIFICATION OF CORRECTIONPatent Noi,v .39010,017 November 217 1961 Wilson M. Brubaker et al.`

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 111 line 7]7 strike ou t "fringing" first, occurrenceQ7 andinsert the Same after "the'HI in line 13 second occurrence6 Signed andsealed this 24th day of April 1962..

(SEAL) Attest:

EsToN 6 JOHNSON DAVID L. LADD Attesting Officer Commissioner of Patents

